Do you mean a site or do you mean a page? Each page can have different keywords, so that overall the site would have many. For individual pages, it is entirely up to you. Some advice I have seen says 2 or 3 keywords per page but I don't agree with this because I want a page to rank for as many as possible.

There is no limit in Keywords. If you have more number of keyword its good for your website to get good traffic.

WRONG. You should read some articles first.

Got this from google webmaster central bla bla bla :

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Does Google ever use the "keywords" meta tag in its web search ranking?

Response :In a word, no. Google does sell a Google Search Appliance, and that product has the ability to match meta tags, which could include the keywords meta tag. But that's an enterprise search appliance that is completely separate from our main web search. Our web search (the well-known search at Google.com that hundreds of millions of people use each day) disregards keyword metatags completely. They simply don't have any effect in our search ranking at present.

Does Google ever use the "keywords" meta tag in its web search ranking?

Response :In a word, no. Google does sell a Google Search Appliance, and that product has which could include the keywords
meta tag. But that's an enterprise search appliance that is completely separate from our main web search. Our web search
(the well-known search at Google.com that hundreds of millions of people use each day) disregards keyword metatags completely.
They simply don't have any effect in our search ranking at present.btw ... your website will look spammy with too many keywords

Ok, the first point to make here is that the OP asked "How many target keywords should a site have?" and you have interpreted that as meaning "how many should I have in the meta tag", but that wasn't the question, no mention was made of the meta tag. The post you quoted was answering in terms of the question.
The keyword meta tag is immaterial to Google but keywords are not. Anyone who looks at the Google Webmaster Tools can see that a site is analysed by Google to see which words are repeated, and how they should reflect what the site is about.
I will use my own site as an example: it is about real estate, so I use all kinds of words in my content, such as properties, property, houses, home, villa, apartment, flat, house etc etc and also combine this with the name of my town and the country.
Together these make up a load of keyword combinations, and it is very possible to have all of them as "target keywords" without putting them in the meta tag.
In other words, these combinations are what I am looking to be ranked for, and I can write content that flows by using a variety of synonyms so Google can see that they are relevant to what my site is about, it doesn't have to look spammy and can be helpful to the user.